This invention relates to wire coil screw thread inserts for tapped holes of workpieces and, more particularly, to an improved tangless wire coil insert.
The conventional wire coil screw thread insert is provided, at one of its ends, with a diametrical tang of its end convolution so arranged that the tang can be gripped by a special tool projected through the coil for insertion of the wire coil insert into a tapped hole. The tang must be broken off and removed after the insertion of the coil insert because the tang would prevent a bolt from being screwed through the coil. Accordingly, a notch is provided on the inside of the end convolution of the coil so that the tang of the coil insert may be broken off. It is evident from such previously known devices, that the broken tang must be accounted for in certain critical applications, e.g. in electronic equipment in which the loose metal tang could cause an electrical short or in internal combustion engines where the loose metal tang could cause engine failure. Further, the extraction of prior art inserts required the use of an extracting tool which cut into the inner threads of the insert and thus rendered the insert unusable for further service.
Further, the problem with prior art inserts, such as tangless inserts as described in U.S. Pat. No. 2,363,789 is that they require the one end convolution having the recessed hook means therein to project inwardly to avoid the possibility of causing burrs or otherwise injuring the tapped thread during the insertion of the helically coiled insert. Also, the extraction of these tangless coil inserts which must be done by using a knife-edge tool damages the inner threads so that the insert cannot be reused.
The method used for inserting a prior art wire coil insert into a tapped hole required an operator to manually orientate the insert to be able to apply a torque to the lead end of the coiled insert during the insertion, since the tang or recessed hook means was only at one end of the insert.
Thus, there exists a need for a double-ended tangless wire coil screw thread insert which can be inserted into the parent material in either direction, without regard to orientation, and can be subsequently removed without damage to the insert and does not require the accountability of broken tangs.